This invention relates to hydraulic actuation systems and more particularly to a hydraulic actuation system for a swing-arm grappler of a gantry crane.
Gantry cranes are commonly used in ports, rail yards or other intermodal shipping facilities for lifting and moving objects such as containers and truck trailers. Such cranes are equipped with various grappler mechanisms to accommodate certain container configurations and associated standard latching systems. For example, highway trailers are typically lifted with a grappler having a swing-arm mechanism, and a standard shipping container typically has four twistlock latches located at the upper four corners of the container for lifting with a grappler having a plurality of corresponding twistlocks. Some grapplers are equipped with both swing arms and twistlocks for selective use as appropriate.
A conventional swing arm grappler includes a platform which is movably suspended from upper beams of the gantry crane and two pairs of arms pivotably mounted to the platform. The arms are configured to extend downwardly from the platform along opposite sides of the trailer. A lower end of each of the arms includes a lifting shoe which extends inwardly to reach under and engage a bottom rail of the trailer for lifting.
For pivoting the swing arms, the conventional gantry crane further includes an hydraulic actuation system adapted to move the arms to either an open, unclamped position, in which each of the arms is upwardly pivoted free from the trailer, or a closed, clamped position, in which the arms are pivoted inwardly to engage and lift a trailer from its bottom rail.
The grappler platform is suspended from a trolley mechanism which is movable in a side-to-side or transverse direction along horizontal beams of the gantry crane. When the grappler is holding an elevated object, such as a trailer or shipping container, acceleration and deceleration of the trolley in a transverse direction results in xe2x80x9cswayxe2x80x9d forces tending to cause the grappler and lifted trailer to swing like a pendulum. The sway motion occurs at the pivot points where the swing arms meet the base.
Unfortunately, conventional hydraulic circuits allow a significant degree of arm sway with a low degree of oscillation decay. Significant sway leads to various problems. For example, the crane operator may have difficulty controlling and positioning a trailer held by swaying grappler arms. In some instances, such swaying can cause the elongate portion of one or more of the arms to be in damaging contact against the lifted trailer. Additionally, crane operation efficiency is diminished because the crane operator must wait for sway motion to adequately decay before continuing, thereby increasing the time per loading or unloading of a container. The swaying motion of the swing arms further results in a rocking action of the respective shoes on the bottom of the trailer, which can damage the trailer and destabilize the lifting contact. Accordingly, a need exists for a hydraulic swing arm actuator which provides improved sway dampening.
The present invention provides an improvement to a hydraulic swing arm actuation circuit for a grappler. The circuit generally includes at least one hydraulic cylinder mounted to move each of the swing arms between clamped and unclamped positions. The circuit includes conduits which direct pressurized fluid as desired to opposite ends of each cylinder to actuate piston movement in a desired direction. The circuit may be part of a closed-loop system driven by a master hydraulic pump which operates other hydraulic features of the gantry crane. In an embodiment according to teachings of the invention, the hydraulic circuit is equipped with at least one dampener which limits flow to dampen sway while not creating a back pressure that would interfere with the flows needed for actuating motion of the grappler arms. The dampener includes a flow restrictor which, in various embodiments, may be an orifice and/or a counterbalance valve to restrict flow to or from the actuators so that swaying motion decays more quickly.
Additionally, according to an embodiment, one-way, non-restricted flow is permitted in an opposite flow direction to circumvent the restrictor. As a result, the arms are only dampened in an outwardly swaying motion, and so that no dampening is applied to the arms when swaying inwardly. This advantageously enhances the lifting contact of the arms and associated lifting shoes against the load.
An advantage of the present invention is that it provides an improved hydraulic circuit for actuating grappler arms.
Another advantage of the present invention is that it provides a hydraulic system which reduces sway motion of grappler arms.
A further advantage of the present invention is that it provides a hydraulic system for a grappler which allows the grappler to be more easily controlled.
Yet another advantage of the present invention is that it provides a hydraulic system for a grappler which reduces damage to trailers.
Additional features and advantages of the present invention are described in, and will be apparent from, the following description, claims and figures.